resolution brush worse than resolution terrain

Creating a 3000m x 3000m terrain, I am struggling with the brush that in its smallest size is a square of about 5m x 5m. So if I want to excavate a small ditch or small sandy path, this will have a minimum width of 5m and the contours become rough triangles !

Of course I can improve this by increasing the height map resolution of the terrain. This is however totally unnecessary, because the terrain's resolution is OK, as I can see after importing the heightmap map, all details show nicely. It is just the resolution of the brush itself that is much worse than the resolution of the terrain.

Can someone tell me how to change this ?